1. Field of the Invention
The present invention refers to an apparatus for measuring the volume of flowing media as well as to a corresponding method and, in particular, it refers to an apparatus for measuring the volume of fuel in gasoline pumps of motor vehicle filling stations.
2. Description of the Related Art
A known type of apparatus for such volume measurements comprises a housing including at least two partially merging longitudinal bores, the axes of which are parallel, in which multiple-thread, interengaging screw spindles are supported in a freely rotatable manner. Said apparatus further comprising at least one sensor by means of which the number of revolutions of at least one of the spindles is detected. Examples of such apparatuses are known from EP 0 572 621 B1, DE 195 13 781 A1, DE 44 23 461 as well as from DE 42 08 869 A1.
One problem arising in connection with screw spindle volumeters is that, taking as a basis the number of spindle revolutions detected by the sensor in the course of a fuelling process, it is not possible to draw exact conclusions with regard to the actually tanked fuel volume. This is especially due to the fact that, when the flow of fuel increases or decreases at the beginning or at the end of a fuelling process, the amount of fuel flowing through the volumeter is larger than the actual amount detected on the basis of the respective number of spindle revolutions. The cause of this behavior is a production-dependent, non-avoidable gap between the spindles and the walls of the longitudinal bores so that parts of the tanked fuel volume can pass through the volumeter without causing a spindle revolution that corresponds to these volume fractions.
On the basis of FIG. 2, the resultant effect on the accuracy of the volume measurement is shown in a graphical representation.
Over the abscissa of the diagram shown, the amount of fuel flowing through the volumeter is shown in the unit l/min. Along the ordinate, the error is plotted in percent, which originates from the deviation of the actually tanked fuel amount from the measured fuel amount. At the zero passage of curve K (point I), a condition has been reached where the measured fuel volume corresponds to the actually tanked fuel volume. At the beginning of the fuelling process, i.e. in a region on the left-hand side of the zero passage (point I), a negative error is caused, said negative error expressing the magnitude of the amount of tanked fuel that could not be detected by the sensor although it flowed through the volumeter. At very high flow velocities, i.e. from point III onwards, for example, the measurement takes place with an approximately constant small positive error, i.e. the amount of fuel measured is only slightly larger than the amount that actually flowed through the volumeter. Between points I and III, an increase in the positive error appears, the excessive amount displayed per volume unit tanked reaching its maximum at point II.
In order to cope with this situation, it has been suggested in EP 0 572 621 B1 that the pulses provided by the sensor should be changed in frequency during the fuelling process in a pulse shaper stage in dependence upon the flow velocity of the fuel, i.e. in dependence upon the location on the abscissa of FIG. 2, so as to achieve a linearization before the pulses reach the actual counter by means of which a total count is ascertained, said total count being converted into a tanked (measured) fuel volume via a fixed relation. It follows that, according to the teaching of EP 0 572 621 B1, pulses supplied by the sensor are still counted, although these pulses are changed on their way to the counter within the pulse shaper stage so as to achieve the above mentioned linearization.